Agree with most of what you said, will just add real fast that I don't think it is possible to get the Mark scene if you haven't told Holly to keep going to the gym, which removes nearly all of Ivy's interactions with her. Until the "no-gym" route gets an equivalent scene, I wouldn't say Lena is "corrupting" Holly, at most she accidentally makes her more influenceable and then lets Ivy do her thing, either because Lena agrees with Ivy's goals (if not her methods) or is too passive about the whole affair.
You are right about Holly not being able to hook up with Mark without going to the gym. But it doesn't necessarily have to do with Ivy's mean and blunt ideas influencing Holly but Holly and Ivy being friends. Mark is Ivy's friend, so of course Holly needs to go to the gym to be introduced to him, otherwise Ivy doesn't really give a damn about Holly. Even if they hung out that time on the path where Holly didn't go to the gym and Holly started wallowing in self-pity, Ivy would just dismiss her and try to change the topic instead of giving her any ideas.
On top of that,
encourage_holly variable is what determines Holly's starting change, representing Holly's growing confidence. If Lena doesn't encourage her to go to the gym, the value for that variable decreases instead of staying the same, thus telling us that Holly is being discouraged by Lena from putting an effort. And if she's not willing to put some effort, she wouldn't even consider Ivy's ideas if Ivy brought them up.
Now, regarding her interactions with Ivy and how they contribute to her corruption (hooking up with Mark) vs Lena's influence.
To hook up with Mark, Holly needs:
1) holly_guy = 2 which is triggered automatically if Lena encouraged Holly to hang out with him and she didn't have sex with Holly, in which case it's only = 1 or 0 (if Lena tried to discourage Holly from meeting him in Ch 7). Lena can try to interrupt them from talking but it only works if holly_change < 4. If it's > 3, Holly prefers Mark's company and wants to keep chatting with him.
2) holly_change = 5 to agree to it if Lena is passive and allows her to make her own decision ("Let Holly decide"). Otherwise she rejects the idea despite Ivy's arguments. However, Lena can push her towards this idea, thus convincing Holly to go for it regardless of holly_change value. So Holly values Lena's input before doing anything Ivy tells her to do.
3) If Lena decides to protect Holly, regardless of other conditions, it completely stops Holly's interest in Mark (holly_guy = 0), even if she hooked up with Ivy and generally listens to her ideas.
So, if we ignore Lena suddenly protecting Holly here or pushing her (which could be argued as joint effort with Ivy rather than Holly listening exclusively to Lena). That leaves us with holly_guy = 2 and holly_change > 3 or holly_change > 5. Let's compare how many holly_change points are contributed by Ivy vs Lena vs Ian. Any point rewarded by Lena agreeing with Ivy or being passive in a situation where Ivy suggests an idea will be attributed to Ivy for the sake of fairness. It somewhat diminishes Lena's agency but whatever.
Ch 1:
- "Yes, she is" +1 to Ian
Ch 6 (holly_gym):
- "Comfort Holly / Let Ivy talk to her" +1 to either Lena / Ivy
- "Agree with Ivy" +1 to Ivy
Ch 7 (holly_gym):
- "She's a writer" +1 (3 max) to Lena - when talking to Marcel
- "Encourage Holly" +1 (3 max) to Ivy - when Ivy shows Mark's photo to Holly
Ch 8 (holly_gym):
- "Give me something strong" +1 (2 max) to Lena - when drinking at Ivy's place
- "Stay at Ivy's place and have another drink" +1 (1 max) to Lena
- "Strip to underwear" +1 (3 max) to Ivy (her idea)
Ch 10:
- "Nudge Holly" +1 (5 max) to Lena
- "Leave them to it" +1 (5 max) - This is tricky because it's Lena's decision to let them talk but Holly in this instance ends up being corrupted from talking to Mark with Ivy doing her own thing elsewhere. Unless you count Mark as an extension of Ivy, but it would completely rob Lena and Holly from agency because Ivy can't influence Holly enjoying Mark's company. Since Ivy is not directly influencing Holly here let's just not count it and award it to Holly herself.
Results:
1) Pitiful 1 point from Ian at the very start of the game. He gets to contribute more in his own path but since it's at odds with Holly hooking up with Mark I ignored those. Also he gets to contribute once more in Ch 10 but only if Holly doesn't go to gym so it's irrelevant here either.
3) Ivy: 3 (1 bonus that can't go beyond 3)
2) Lena: 3 (1 bonus that can't go beyond 3 and another that applies only if the score was 0).
So, we can see that Lena's and Ivy's influence rather match and Holly can only be corrupted by joint effort but it's besides the point. We only need Holly to hook up with Mark without choosing anything that would increase holly_change by Ivy's influence.
1) +1 "Yes, she is" by Ian
2) +1 "Comfort Holly" by Lena
3) +1 "She's a writer" by Lena
4) +1 "Nudge Holly" by Lena
5) +1 "Leave them to it" - passed the required threshold
And thus, Ian and Lena can contribute to Holly hooking up with Mark without Ivy having anything to do with it aside from orchestrating their interaction on her birthday. While sure, none of these options can really be attributed to corruption as on their own they're quite innocent and Holly simply becomes more confident in herself. But it's Lena helping Holly with her confidence in order to loosen up. It's not fair to say that she simply lets Ivy to do her thing because she can still reject some of Ivy's ideas and only go along with those that she thinks benefit Holly. Ivy simply can't corrupt Holly if Lena is being passive. Holly needs Lena's encouragement in order to be brave enough to go the extra mile.
Lena being protective of Holly in regards to Ian was less about the corruption and more about the heartbreak. I think that if you play your Lena to not be interested in romance anymore, her behavior in regards to pushing Holly into a one-night stand with Mark isn't really contradictory nor a sign of Lena's own corruption. Lena starts the game at a crossroads and uncertain about how she feels about the entire idea of finding love due to the heartbreak she had with Axel, she can simultaneously think the one-night stand with Ian was a bad idea because it would bring Holly pain while also think that the one-night stand with Mark is not a terrible idea because being able to have casual relationships without romantic feelings would be a good thing for Holly (although I think that no matter the scenario, Lena is always a bit shocked about Holly giving Mark a bj in their first date, rathen than impressed or proud)
Only, how can Lena be sure that Holly doesn't suffer heartbreak from Mark if she learned that he was simply playing with her because Ivy told him to? There's no confirmation that Holly simply uses Mark to get sexual experience. Sure, that's what Ivy tells her to do, but Holly can actually grow to enjoy talking with Mark and for all Lena knows, she could develop an equally strong crush just like she did with Ian. We don't know with what mindset Holly pursues her relationship with Mark. Sure, she partially wants to perform Ivy's tasks and gain more sexual experience, but in the path where she dates Ian, she can be doing the exact same thing with him, opening up to be more flirtatious and kinky, sending nudes, using dirty talk, etc. On top of it, Holly can relatively quickly fall in love with Lena after being rejected by Ian if they end up having sex. So with Mark she can feel similar emotions, using him to forget about Ian but simultaneously developing an equally strong crush.
The only branch where Lena is somewhat aware that Holly's fling with Mark is casual is if she had Holly had sex with either Lena or Ivy. In this branch she knows that by hooking up with Mark Holly will simply get more experience and that after tasting him she won't suddenly stop sleeping with Ivy or Lena. But if Holly has been dealing with her heartbreak on her own, Lena can't really be sure that Holly won't catch feelings for Mark or doesn't already have them. But she knows that Holly is a sweet and romantic girl, so it's weird that she wasn't as protective of her dating Mark, and regardless of how Ivy might present it and if Mark can deny that they have anything serious, Holly can see it as a getting-to-know phase, expecting something more in the future. And while we know that she probably won't be heartbroken if Mark treats her as a fuck buddy because unlike Louise she doesn't assume anything of the relationship before talking about it, but if Lena was so protective of Holly towards Ian, she most likely thinks that Holly can't handle casual relationships. With Ian, her arguments were mostly "But Holly is different", and not "She had a crush on you!". So what exactly happened that made Lena believe that Holly is not that different anymore and can now be okay with casual sex?
Also I think it is important for there to be a distinction between "corrupting" someone and just sharing your own opinion and being a part of another person's growth. Saying that any kind of influence is a form of corruption just because it resulted in change is a dangerous line of reasoning in my opinion, hence why I associate "corruption" more with coercion and manipulation than just the result of change.
I agree that not all influence is corruption especially if it's well-intentioned and mostly wholesome, but as I said earlier, corruption can be unintentional and accidental because it's not really about actions but the outcome, and can happen even without any active influence of the protagonist (or even other NPCs). Would you disagree that in GGGB Eva had a corruption arc? Because it was only triggered by Ash not warning her about Jack (a passive choice), and Jack didn't really coerce or manipulate Eva in any way, he simply hit on her and let her do what she wanted without any judgment. And with that, she dove head-first into cheating, drugs, hanging out with thugs, etc, completely on her own (well, with the help of Jack and later thugs that benefited from her thrill-seeking).